Semiconductor device with a cavity therein and a method of manufacturing the same

ABSTRACT

A semiconductor device includes a semiconductor substrate, cavities, and an element isolating region. The cavities, which are each shaped like a flat plate, are made in the semiconductor substrate. The element isolating region is formed in the surface of the semiconductor substrate and located at the sides of the cavities.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-273409, filed Sep. 19,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor device and a method offabricating the semiconductor device. More particularly, this inventionrelates to techniques used for system LSIs using SON (Silicon onNothing) substrates.

2. Description of the Related Art

It is well known that an SOI (Silicon on Insulator) has a structurewhere a silicon layer is formed on an insulating film. Formingsemiconductor elements on such an SOI enables semiconductor integratedcircuits to consume less electric power and operate at higher speeds.Methods of forming SOIs include a method of laminating two substratestogether and an SIMOX (Separation by Implanted Oxygen) method. SOIs,however, have the disadvantages that they are higher in manufacturingcost and that it is difficult to form a silicon layer with defect-free.

With this backdrop, an SON structure where a silicon layer is providedat the top of a cavity has lately attracted attention. It is safe to saythat the SON is the final SOI structure. The SON has the same merits asthose of the SOI. The SON is currently under intensive investigation.For example, a method of insulating a silicon layer from a semiconductorsubstrate has been disclosed in Jpn. Pat. Appln. KOKAI Publication No.5-206257. An SON manufacturing method capable of micro-fabrication hasbeen disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-288381. Amethod of manufacturing double-gate MOS transistors using SONs has beendisclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-257358. Aninfrared sensor using SON has been disclosed in Jpn. Pat. Appln. KOKAIPublication No. 2001-281051. From these, it can be seen that the studyof SON has covered various fields.

The configuration of a conventional semiconductor device using an SONstructure will be explained by reference to FIG. 1 (for further details,refer to, for example, Jpn. Pat. Appln. KOKAI Publication No.2001-144276). FIG. 1 is a sectional view of a MOS transistor formed onan SON substrate.

As shown in FIG. 1, cavities 110 are made in a semiconductor substrate100. Then, source and drain regions 120, 120 are formed in an elementregion AA10 located at the top of the cavity 110. A gate electrode 140is formed on the element region AA10 with a gate insulating film 130interposed therebetween, thereby forming a MOS transistor. A sidewallinsulating film 170 is formed on the sidewall of the gate electrode 140.Adjacent MOS transistors are electrically separated from one another byan element isolating region 150 formed between them. The elementisolating region 150 is formed normally using STI (Shallow TrenchIsolation) techniques from the viewpoint of the micro-fabrication ofelements.

As described above, the use of SON has been encouraging an attempt tocause semiconductor integrated circuits to consume less power andoperate at higher speeds. It is expected that SON will be applied tosystem LSIs embedded, for example, DRAM (Dynamic Random Access Memory)in the future.

The conventional SON structure, however, tends to make themicro-fabrication of semiconductor devices difficult.

Specifically, as shown in FIG. 1, when SON and STI techniques are used,the element isolating region 150 has to be prevented from exposing tothe cavity 110. The reason is that, in the STI technique, trenches areformed in a semiconductor substrate and then, the trenches are filledwith an insulating film, thereby forming an element isolating region. Ifthe trenches are exposed to the cavities 110, what supports the elementregion AA10 will be lost. To avoid this problem, a distance of, forexample, d1 is allowed between the cavity 110 and the element isolatingregion 150. Depending on the situation, the clearance makes a totallyuseless region, which contributes to an increase in the element area.

Furthermore, in the conventional structure, the region between thecavities 110 and the element isolating region 150 electrically connectsthe element regions AA10 with the semiconductor substrate 100. To solvethis problem, well regions 160, which are unnecessary in the SOIstructure, must be used to electrically separate the element region AA10from the semiconductor substrate 100. As a result, it is difficult tonarrow the distance between adjacent semiconductor elements, which mayinterfere with the micro-fabrication of semiconductor devices.

BRIEF SUMMARY OF THE INVENTION

A semiconductor memory device according to an aspect of the presentinvention, comprises: a semiconductor substrate; a flat-plate-shapedcavity made in the semiconductor substrate; and an element isolatingregion formed in the surface of the semiconductor substrate and locatedat the sides of the cavity, the cavity being wider than an elementregion provided on the cavity, wherein the element isolating region isless deep than the cavity and deeper than the element region.

A method of fabricating a semiconductor device according to anotheraspect of the present invention comprises:

making flat-plate-shaped cavities partly in a semiconductor substrate;

forming an insulating film in the surface of the semiconductor substratebetween adjacent ones of the cavities in such a manner that a part ofthe insulating film is exposed to the cavities so as to electricallyseparate element regions provided on the adjacent cavities from eachother each cavity being wider than each element region; and

forming semiconductor elements on the element regions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view of a conventional semiconductor device;

FIG. 2A is a plan view of a semiconductor device according to a firstembodiment of the present invention;

FIG. 2B is a sectional view taken along line 2B—2B of FIG. 2A;

FIG. 3A to FIG. 3I are sectional views successively showing themanufacturing steps of the semiconductor devices according to the firstembodiment;

FIG. 4A is a plan view of a semiconductor device according to a secondembodiment of the present invention;

FIG. 4B is a sectional view taken along line 4B—4B of FIG. 4A;

FIG. 5A to FIG. 5E are sectional views successively showing themanufacturing steps of the semiconductor devices according to the secondembodiment;

FIG. 6A to FIG. 6C are sectional views successively showing themanufacturing steps of the semiconductor devices according to amodification of the second embodiment;

FIG. 7A is a plan view of a semiconductor device according to a thirdembodiment of the present invention;

FIG. 7B is a sectional view taken along line 7B—7B of FIG. 7A;

FIG. 8A is a sectional view of a part of a semiconductor deviceaccording to the first to third embodiments; and

FIG. 8B is a sectional view showing a part of the manufacturing step ofthe semiconductor devices according to the first to third embodiments.

DETAILED DESCRIPTION OF THE INVENTION

A semiconductor device according to a first embodiment of the presentinvention and a method of manufacturing the semiconductor devices willbe explained by reference to FIG. 2A and FIG. 2B. FIG. 2A is a plan viewof MOS transistors using SON. FIG. 2B is a sectional view taken alongline 2B—2B of FIG. 2A.

As shown in FIGS. 2A and 2B, semiconductor layers 12 are formed above asemiconductor substrate 10. Flat-shaped cavities 11 are located betweenthe semiconductor substrate 10 and semiconductor layers 12. The area ofthe surface (or back surface) of the semiconductor layer 12 is smallerthan the area of the top (or the bottom) of the cavity 11. Thesemiconductor layer 12 overlaps with the cavity 11. That is, the backsurface of the semiconductor layer 12 is exposed to the cavity 11.Around the semiconductor layer 12, an element isolating region 13 isformed. The element isolating region 13 is also formed in thesemiconductor substrate 10 between adjacent cavities 11 in such a mannerthat the region 13 is located at the sides of the cavities 11 in thesemiconductor substrate 10. A part of the element isolating region 13 isexposed to the cavity 11. Consequently, the semiconductor layer 12 hasits bottom surface surrounded by the cavity and its side faces enclosedby the element isolating region 13, thereby being electrically separatedfrom the semiconductor substrate 10. The element isolating region 13 isformed so as to be less deep than the bottom of the cavity 11 and deeperthan the top of the cavity 11 from the surface of the semiconductorsubstrate 10.

In the semiconductor layer 12, source and drain regions 14, 14 are soformed that they are separated from each other. A gate electrode 16 isformed on the semiconductor layer 12 between the source and drainregions 14, 14 with a gate insulating film 15 interposed therebetween. Asidewall insulating film 17 is formed on the sidewall of the gateelectrode 16.

The gate electrode 16 is extended in a specific direction and has itsend drawn across the semiconductor layer 12 onto the element isolatingregion 13. Then, a potential is applied to the gate electrode 16 at acontact region 18 formed on the element isolating region 13.

As described above, a MOS transistor including the source and drainregions 14, 14, gate insulating film 15, and gate electrode 16 is formedon the SON structure.

Next, a method of manufacturing the semiconductor device will beexplained by reference to FIG. 3A to FIG. 3I. FIG. 3A to FIG. 3I aresectional views successively showing the manufacturing steps of thesemiconductor device.

As shown in FIG. 3A, a mask material 20 is formed on the semiconductorsubstrate (e.g., a silicon substrate) 10. The mask material 20 is, forexample, a silicon oxide film or a multi-layer including the siliconoxide film and silicon nitride film. Next, a photo-resist 21 is coatedon the mask material 20. Then, the photo-resist 21 is patterned byphotolithography as shown in figure.

Next, the mask material 20 is etched by anisotropic etching, such as RIE(Reactive Ion Etching) using the photo-resist 21 as a mask. As a result,the mask material 20 is patterned the same as the photo-resist 21, asshown in FIG. 3B.

Then, the photo-resist 21 is removed by ashing method. As shown in FIG.3C, with the patterned mask material 20 as a mask, the semiconductorsubstrate 10 is etched by RIE, thereby forming a plurality of trenches21. Consequently, it is desirable that the material which has largedifference in etching rate with silicon is used as mask material 20. Forexample, the silicon oxide film or multi-layer including the siliconoxide film and silicon nitride film is used. The trenches 22 are formedin regions where the cavities 11 are to be made. The radius of anopening, depth of the trench and distance between adjacent trenches areabout 0.2 μm, 2 μm and 0.7 μm, respectively. For making a cavity 11, thedistance D between adjacent trenches is set at a value given by thefollowing equation:D<3.5Rwhere R represents the radius of the opening of the trench.

Next, the mask material 20 is removed. Then, high-temperature annealingis executed at a temperature of about 1,100° C. in a non-oxidizingatmosphere, such as 100% Hydrogen, under low-pressure of 10 torr. As aresult, cavities 23 are made a result of the openings of the trenches 22being closed as shown in FIG. 3D. The annealing is further continued,thereby integrating the cavities 23 into units, which makesflat-plate-shaped cavities 11 as shown in FIG. 3E. This phenomenon iscaused by the surface migration of the silicon to minimize a surfaceenergy of the silicon substrate after removing the silicon oxide film(mask material 20) on the silicon substrate.

Next, an element isolating region 13 is formed by LOCOS (LOCal Oxidationof Silicon) method. Specifically, as shown in FIG. 3F, a mask material,such as a silicon oxide film 24 and silicon nitride film 25, is formedon the semiconductor substrate 10 by CVD (Chemical Vapor Deposition)method. Then a photo-resist 26 is coated on the silicon nitride film 25.Thereafter, the photo-resist 26 is patterned by photolithography asshown in figure. That is, the photo-resist 26 on the regions where theelement isolating region 13 is to be formed is removed. The region wherethe element isolating region 13 is to be formed is the region which isbetween adjacent cavities 11 and region which surrounds a part of thesilicon substrate on the cavities 11.

Next, the silicon nitride film 25 and silicon oxide film 24 issequentially etched by RIE using the photo-resist 26 as a mask. As aresult, the silicon nitride film 25 and silicon oxide film 24 ispatterned the same as the photo-resist, as shown in FIG. 3G. After that,the photo-resist 26 is removed by ashing method.

Then, the surface of the silicon substrate 10 exposed with the patternedsilicon oxide film 24 and silicon nitride film 25 is oxidized by, forexample, wet oxidizing method. As a result, the silicon oxide filmobtained by oxidizing the semiconductor substrate 10 forms an elementisolating region 13. At this time, the element isolating region 13 is soformed that it reaches the cavities 11.

Next, the silicon oxide film 24 and silicon nitride film 25 are removed.As a result, a SON substrate as shown in FIG. 3I is completed. Let theregion above the cavity 11 in the semiconductor substrate 10 be referredto as a semiconductor layer 12. Then, the semiconductor layer 12overlaps completely with the cavity 11, with the back surface of thelayer 12 exposed to the cavity 11. That is, the area of the top of eachcavity 11 is larger than the area of the back surface of thesemiconductor layer 12. The side face of the semiconductor layer 12 isenclosed by the element isolating region 13. Consequently, thesemiconductor layer 12 is electrically separated from the semiconductorsubstrate 10.

Thereafter, source and drain regions 14 are formed in the semiconductorlayer 12 and gate electrodes 16 are formed on the semiconductor layer 12using well-known techniques, which completes a MOS transistor shown inFIGS. 2A and 2B.

As described above, in the semiconductor device and the method ofmanufacturing the semiconductor device in according to the firstembodiment of the present invention, the element isolating region 13 isformed in the surface of the silicon substrate between adjacent ones ofcavities 11 by LOCOS techniques. Therefore, unlike the use of STItechniques, the use of LOCOS techniques enables the element isolatingregion 13 to be formed in such a manner that it is located at the sidesof the cavities 11. Consequently, a part of the element isolating region13 is exposed to the sides of the cavity 11. That is, there is no needto allow a useless space between the cavities 11 and the elementisolating region 13. In other words, the semiconductor layers 12 aresupported by the element isolating regions 13. Moreover, the lowersurface of each semiconductor layer 12 is exposed to a cavity 12 and theelement isolating region is so formed that reaches the cavity fromsurface of the silicon substrate. Consequently, the cavities 11 andelement isolating region 13 electrically separate the semiconductorlayers 12 from the semiconductor substrate 10, so that well separationis not necessary differently to conventional equivalents. Therefore, thearea occupied by semiconductor elements can be reduced and therefore thesemiconductor device can be further miniaturized. This leads to areduction in the manufacturing cost of semiconductor devices.

If a semiconductor layer 12 is defined as one element region AA, theelement region AA and a cavity 11 are formed in a one-to-onecorrespondence. That is, the cavity 11 is nearly as wide as the elementregion AA, which means that a relatively small size of the cavity issufficient. Therefore, the processes of manufacturing semiconductordevices can be simplified and the manufacturing yield can be improved.While in the first embodiment, one MOS transistor has been formed in oneelement region AA (or semiconductor layer 12), a plurality ofsemiconductor elements electrically connected may be formed in oneelement region AA.

A semiconductor device according to a second embodiment of the presentinvention and a method of manufacturing the semiconductor devices willbe explained by reference to FIG. 4A and FIG. 4B. FIG. 4A is a plan viewof MOS transistors using SON. FIG. 4B is a sectional view taken alongline 4B—4B of FIG. 4A.

As shown in the figures, a semiconductor device according to the secondembodiment includes an SON region where a cavity 11 is made and a bulkregion where no cavity 11 is made. Since the structure of the SON regionis the same as in the first embodiment, its explanation will be omitted.

In the bulk region, a well region 30 is formed in the surface of thesemiconductor substrate 10. In the surface of the well region 30, sourceand drain regions 31, 31 are so formed that they are separated from eachother. On the well region 30 between the source and drain regions 31,31, a gate electrode 33 is formed with a gate insulating film 32interposed therebetween. On the sidewall of the gate electrode 33, asidewall insulating film 34 is formed.

A MOS transistor in the bulk region formed as described above isenclosed by an element isolating region 13 and an element isolatingregion 35. The gate electrode 33 is extended in a specific direction andhas its end drawn across the well region 30 onto the element isolatingregion 35. Then, a potential is applied to the gate electrode 33 at acontact region 36 formed on the element isolating region 35.

Next, a method of manufacturing the semiconductor device will beexplained by reference to FIG. 5A to FIG. 5E. FIG. 5A to FIG. 5E aresectional views successively showing the manufacturing steps of thesemiconductor device according to the second embodiment.

According to the manufacturing steps explained in the first embodiment,a cavity 11 is made and an element isolating region 13 are formed in anSON region in the semiconductor substrate (e.g., a silicon substrate)10. That is, as shown in FIG. 5A, trenches 21 are made in the SON regionof the semiconductor substrate 10. These trenches 22 are formed in theregions where cavities 11 are to be made.

Next, as shown in FIG. 5B, the mask material 20 is removed andhigh-temperature annealing is done in a non-oxidizing atmosphere. As aresult, a flat-plate-shaped cavity 11 as shown in the figure is made inthe SON region.

Then, as shown in FIG. 5C, an element isolating region 13 is formed inthe SON region by LOCOS techniques. The element isolating region 13 isso formed in the semiconductor substrate 10 that it is located at thesides of the cavity 11. Therefore, a part of the element isolatingregions 13 is exposed to cavity 11.

Next, an element isolating region 35 is formed in the bulk region of thesemiconductor substrate 10 by STI techniques. That is, a mask material40 is first formed on the semiconductor substrate 10 by CVD techniques.The mask material 40 is, for example, a silicon oxide film or a siliconnitride film. Then, the mask material 40 is patterned byphotolithography and RIE, thereby removing the mask material 40 in theregion where an element isolating region 35 is to be formed.Furthermore, with the patterned mask material 40 as a mask, thesemiconductor substrate 10 is etched, thereby forming a trench 41 in thebulk region of the semiconductor substrate 10 as shown in FIG. 5D.

Next, the trench 41 is filled with an insulating film, such as a siliconoxide film and the mask material 40 is removed, thereby forming anelement isolating region 35.

Therefore, a well region 30 is formed in the bulk region by ionimplantation techniques or the like. Furthermore, a MOS transistor isformed in each of the SON region and bulk region by a well-known method,which completes a structure shown in FIGS. 4A and 4B.

In the semiconductor device and the method of manufacturing thesemiconductor device according to the second embodiment, the elementisolating region 13 in the SON region has been formed by the LOCOStechniques. Consequently, the same effect as that of the firstembodiment is obtained.

In the structure according to the second embodiment, the SON region witha cavity 11 and the bulk region with no cavity 11 are made in the samesemiconductor substrate. The reason for this will be explained.

In a system LSI, whether the use of SON is good or not varies fromcircuit to circuit. This is because the SON has a floating body effectinherent therein. Specifically, since the semiconductor layer 12 in theSON region is electrically isolated from the semiconductor substrate 10,the potential of the semiconductor layer 12 is floating. Thus, it isdesirable that a semiconductor element that carries out digitaloperations should be formed on the semiconductor layer 12. On the otherhand, it is undesirable that a semiconductor element that carries outanalog operations should be formed on the semiconductor layer 12 whosepotential is unstable.

With the second embodiment, whether the MOS transistor on the SON (orsemiconductor layer 12) or the MOS transistor on the semiconductorsubstrate 10 is used can be determined, depending on the characteristicsof the semiconductor element. Thus, it is possible to realize ahigh-speed, high-performance system LSI.

The method of forming the SON region and bulk region in the samesemiconductor substrate 10 is not limited to the method shown in FIGS.5A and 5B. FIGS. 6A to 6C are sectional views successively showing themanufacturing steps of the semiconductor device according to amodification of the second embodiment.

As shown in FIG. 6A, an element isolating region 35 is formed in thebulk region by STI techniques. Then, as shown in FIG. 6B, aflat-plate-shaped cavity is formed in the SON region. Thereafter, asshown in FIG. 6C, an element isolating region 13 so formed in thesemiconductor substrate 10 that located at the sides of the cavity 11.

A semiconductor device according to a third embodiment of the presentinvention will be explained by reference to FIG. 7A. The thirdembodiment is such that the second embodiment is applied to a system LSIembedded DRAM. FIG. 7A is a plan view of a system LSI embedded DRAM.

As shown in FIG. 7A, a DRAM cell array is formed in a bulk region and alogic circuit is formed in an SON region. A dummy pattern for the DRAMcells is formed in the bulk region (hereinafter, referred to as aboundary region) in contact with the SON region.

In the bulk region, a plurality of element regions AA1 are arranged in astaggered manner. The shaded regions in FIG. 7A represent elementregions AA1. Element isolating regions are provided in the regions otherthan the element regions AA1. Each element region is so formed that ithas a length of 4F (F: minimum processing dimension) in the longitudinaldirection and a width of 2F in the direction perpendicular to thelongitudinal direction. The DRAM cell array includes a plurality ofmemory cells, each having a cell transistor provided in an elementregion AA1 and a trench-type cell capacitor TC provided so as to be incontact with both ends of the element region AA1 in the longitudinaldirection. A plurality of bit lines BL electrically connected viabit-line contact plugs BC to the memory cells located in the same columnare arranged along the longitudinal direction of the element region AA1.a plurality of word lines WL electrically connected to the gateelectrodes of the cell transistors in the same row are arranged in thedirection perpendicular to the longitudinal direction of the elementregion AA1.

In the boundary region, element regions AA1 which have the same patternas that of the DRAM cells are formed. These element regions constitute adummy pattern which is not used in forming DRAM cells. In a DRAM or thelike, an enormous number of memory cells are arranged regularly in anarray. Its regularity, however, is disordered at the ends of the DRAMcell array. As a result, the photolithography conditions or etchingconditions at the ends of the DRAM cell array tend to vary, which makesit difficult to maintain the reliability of the memory cells. Toovercome this problem, a dummy pattern having the same pattern as thatof the DRAM cell array is formed outside the DRAM cell array, therebymaintaining the reliability of the memory cells in the DRAM cell array.In the third embodiment, the dummy pattern is formed in the boundaryregion between the bulk region and the SON region.

In the SON region, a logic circuit is formed. An explanation of thelogic circuit will be omitted.

A sectional structure of the system LSI shown in FIG. 7A will beexplained by reference to FIG. 7B. FIG. 7B is a sectional view takenalong line 7B-7B of FIG. 7A. A structure of the DRAM cell array in thebulk region will be first described.

In a p-type silicon substrate 50, a trench 51 for forming a trenchcapacitor TC is formed. A capacitor insulating film 52 is formed on allinner surface of the trench 51 except for an upper portion thereof. Acollar oxide film 53 thicker than the capacitor insulating film 52 isformed on the upper portion of the inner surface of the trench 51, wherethe capacitor insulating film 52 is not formed, and except for theuppermost portion. A storage node electrode 54 is buried partway in thetrench 51. A conductive material layer 55 is further formed on thestorage node electrode 54. Moreover, a low-resistance conductivematerial layer 56 is further formed on the uppermost portion of thetrench 51 near the opening portion. An n⁺-type impurity diffused layer57 is formed in the silicon substrate 50 in such a manner that it is incontact with the capacitor insulating film 52. The n⁺-type impuritydiffused layer 57 functions as a plate electrode. Furthermore, an n-typewell region 58 connected in common to a plurality of n⁺-type impuritydiffused layers 57 is formed in the silicon substrate 50. Thus, atrench-type cell capacitor TC is formed.

A gate electrode 33 is formed on the silicon substrate 50 with a gateinsulating film 32 interposed therebetween. An insulating film 34 is soprovided that it covers the gate electrode 33. N⁺-type source and drainregions 31 are formed in the surface of the silicon substrate 50,thereby forming a cell transistor. The source region 31 of the celltransistor is electrically connected to the conductive material layer 56of the cell capacitor TC. A plurality of units of the DRAM cellincluding the cell transistor and cell capacitor are provided in theDRAM cell array. The DRAM cells are arranged in units of two cells inthe elements AA1 electrically separated by the element isolating regions35. The two DRAM cells in each pair share a drain region 31. The elementisolating region 35 in the bulk region is formed by STI techniques asexplained in the second embodiment.

An interlayer insulating film 60 is formed on the silicon substrate 50to cover the DRAM cells. A bit-line contact plug BC extending from thesurface of the interlayer insulating film 60 to the drain region 31 isformed in the interlayer insulating film 60. A bit line BL electricallyconnected to the bit-line contact plug BC is formed on the interlayerinsulating film 60.

In the boundary region, that is, in the bulk region in contact with theSON region, element regions AA1 which have the same pattern as that ofthe DRAM cells are formed. No semiconductor element is formed there. Ann-type well region 61 connected to the n⁺-type impurity diffused layer57 of the cell transistor is formed in such a manner that it reaches thesurface of the silicon substrate 50. In this region, a plate potentialis applied to the n-type well region 61.

In the SON region, a cavity 11 is made in the surface of thesemiconductor substrate 50 as explained in the first and secondembodiments. A region of the semiconductor substrate 10 located at thetop of the cavity 11 is referred to a semiconductor layer 12. Thesemiconductor layer 12 is enclosed by an element isolating region 13.The element isolating region 13 is, of course, a silicon oxide filmformed by oxidizing the semiconductor substrate 50 by LOCOS techniques.The semiconductor layer 12 is part of the semiconductor substrate 50.Source and drain regions 14, 14 are formed in the semiconductor layer12. A gate electrode 16 is formed on the semiconductor layer 12 with agate insulating film 15 interposed therebetween. In addition, aninsulating film 17 is formed to cover the gate electrode. An interlayerinsulating film 60 is formed on the semiconductor substrate 50.

Then, an interlayer insulating film 62 covers the DRAM cell array, dummypattern, and logic circuit.

As described above, in the semiconductor device according to the thirdembodiment, a circuit required to have high controllability for leakagecurrents or threshold voltages, such as a DRAM cell array or a senseamplifier, is formed in the bulk region, whereas a logic circuit thatcarries out digital operations is formed in the SON region. Therefore,the DRAM cell array, sense amplifier, and logic circuit can be operatedunder the optimum conditions. This enables the system LSI to operate athigher speeds and have better performance.

While in the third embodiment, only the element regions AA1 have beenprovided in the boundary region, trench capacitors may be additionallyformed. Dummy memory cells may, of course, be formed. The embodiment ofthe present invention is not limited to the system LSI embedded DRAM.For instance, the embodiment may be applied to a system LSI thatincludes an SRAM (Static RAM), a flash memory, a Ferroelectric RAM, oran MRAM (Magneto-resistive RAM). The third embodiment is not restrictedto an LSI having a semiconductor memory device. It may be widely appliedto the semiconductor devices including logic circuits and digitalcircuits.

As described above, in the semiconductor devices and the method ofmanufacturing the semiconductor devices according to the first to thirdembodiments, the element isolating region between cavities in the SONstructure has been formed by LOCOS techniques. Moreover, the elementisolating region is provided at and exposed to the sides of thecavities. As a result, not only is a useless space between the cavityand the element isolating region required, but also no well separationis needed. This enables the semiconductor devices to be miniaturizedfurther. Since the size of the cavity is almost the same as that of theelement region, this makes the manufacture easier, which improves theyield in manufacturing semiconductor devices. Furthermore, the SONstructure is used for a circuit that carries out digital operations,whereas bulk silicon is used for a circuit that carries out analogoperations, which enables each circuit to operate under the optimumconditions. This enables the system LSI to operate at higher speeds andhave better performance.

A MOS transistor using SON will be explained by reference to FIG. 8A.FIG. 8A is a sectional view of an SON structure. As shown in the figure,in a MOS transistor of the generation with a minimum processingdimension of 0.1 μm, the width W1 of the semiconductor layer 12 (orelement region AA) is about 5000 to 6000 Å. The thickness d2 of thesemiconductor layer 12 is about less than 500 Å. It is known that thethinner the semiconductor layer 12, the more the short channel effectcan be prevented. Particularly, it is desirable that the thicknessshould be less than ¼ of the gate length.

As shown in FIG. 8B, the film thickness of the semiconductor layer 12 isalmost determined by the width of the opening of the trench 22 forforming a cavity 11. That is, if the width of the opening of the trenchis a1, the film thickness of a semiconductor layer 12 formed bysubsequent annealing is also about a1. Therefore, to make the filmthickness d2 of the semiconductor layer 12 500 Å, the width a1 of theopening of the trench is made 500 Å.

As described above, the film thickness of the semiconductor layer 12provided at the top of the cavity 11 is very small. Accordingly, thedepth of the element isolating region 13 is small. That is, the elementisolating region 13 is nearly as thick as the semiconductor layer 12.Therefore, even if the element isolating region 13 is formed by LOCOStechniques, bird's beaks cause almost no problem and therefore do notprevent the semiconductor devices from being miniaturized further.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A semiconductor device comprising: a semiconductor substrate; aflat-plate-shaped cavity made in the semiconductor substrate; and anelement isolating region formed in the surface of the semiconductorsubstrate and located at the sides of the cavity, the cavity being widerthan an element region provided on the cavity; wherein the elementisolating region is less deep than the cavity and deeper than theelement region.
 2. The semiconductor device according to claim 1,wherein the element isolating region and the cavity enclose the elementregion and electrically separate the element region from thesemiconductor substrate.
 3. The semiconductor device according to claim1, wherein only one element region is provided on the cavity.
 4. Thesemiconductor device according to claim 1, wherein the element isolatingregion is formed of an oxide film obtained by oxidizing thesemiconductor substrate.
 5. The semiconductor device according to claim1, wherein the cavity has no element therein.
 6. The method according toclaim 1, wherein the cavity has no element therein.
 7. A semiconductordevice comprising: a semiconductor substrate; a plurality offlat-plate-shaped cavities made in the semiconductor substrate; and anelement isolating region formed in the surface of the semiconductorsubstrate between adjacent ones of the cavities, a part of the elementisolating region being exposed to the cavities, each cavity being widerthan each element region provided on each cavity, respectively; whereinthe element isolating region is less deep than the cavities and deeperthan the element region.
 8. The semiconductor device according to claim7, wherein the element isolating region and the cavities enclose theelement regions and electrically separate the element regions from thesemiconductor substrate.
 9. The semiconductor device according to claim7, wherein only one element region is provided on the cavity.
 10. Thesemiconductor device according to claim 7, wherein the element isolatingregion is formed of an oxide film obtained by oxidizing thesemiconductor substrate.
 11. The semiconductor device according to claim7, wherein the cavity has no element therein.
 12. A method offabricating a semiconductor device, comprising: making flat-plate-shapedcavities partly in a semiconductor substrate; forming an insulating filmin the surface of the semiconductor substrate between adjacent ones ofthe cavities in such a manner that a part of the insulating film isexposed to the cavities so as to electrically separate element regionsprovided on the cavities from each other, each cavity being wider thaneach element region; and forming semiconductor elements on the elementregions; wherein the insulating film is less deep than the cavities anddeeper than the element region.
 13. The method according to claim 12,wherein the insulating film is formed by oxidizing the surface of thesemiconductor substrate.
 14. The method according to claim 12, whereinthe insulating film and the cavities enclose the element regions andelectrically separate the element regions from the semiconductorsubstrate.
 15. The method according to claim 12, wherein only oneelement region provided on each of the cavities.